BRIDGE ENGINEERING
Design Loads and
Combinations
Prof. Ho-Kyung Kim
How many loads should we
consider for the design of
bridges?
Permanent Loads
l Those loads which always remain and act on a bridge throughout its life
l See Table 3.5.1-1 in AASHTO-LRFD, Table 3.5.1 in KBDC-LSD
l Dead load
l Deck, stay-in-place forms, sidewalks, railings, parapets, primary members, secondary members, stiffeners, signing, wearing surface, utilities (higher load factor with larger uncertainty)
l Superimposed dead load
l Those loads placed on the superstructure after the deck has cured and begun to work with the primary members in resisting loads
l Resisted by a composite section
l Part of dead load, sidewalks, railings, parapets, signing, utilities,wearing surface
l Pressures
l Due to earth or water are also considered permanent loads
Temporary Loads
l Those loads which are placed on a bridge for only a short period of time
l Vehicle live load
l Earthquake loading
l Wind loading
l Channel forces
l Longitudinal forces
l Centrifugal forces
l Impact (Dynamic Load Allowance)
l Construction loads
Deformation and Response Loads
l Deformation loads are those loads induced by the internal or external change in material properties or member geometry.
l Response loads are those loads created by the response of the structure to a given loading condition.
l Creep
l Shrinkage
l Settlement
l Uplift
l Thermal forces
One step further for important
loadings …
Dead Loads
l AASHTO LRFD
Table 3.5.1-1, AASHTO LRFD 2007
(Vehicle) Live Loads (LL)
l A load that moves along the length of a span
l Do not present an actual truck being used to transport goods and materials
l Approximations used to simulate the greatest bending and shear forces caused by actual trucks
l AASHO 1935 truck train loadings (H-20-35)
Fig.3.11 p96
(Vehicle) Live Loads (LL)
l AASHTO Standard truck loading (HS20-44)
(Vehicle) Live Loads (LL)
l AASHTO Standard lane loading (HS20-44)
= 8.165 ton
= 11.793ton
= 6.123 ton
= 8.845 ton
(Vehicle) Live Loads (LL)
l AASHTO LRFD (HL-93 loading)
l Consists of a design truck or tandem (whichever produces the greater forces), combined with a design lane load
l The design truck is identical to HS20-44.
l The design tandem consists of a pair of 111KN axles spaced 1.2m apart.
l The design lane load is 9.34KN/m, used in conjunction with the design truck or tandem.
l Note that in the AASHTO Standard Spec., no lane load is required to be added to an HS truck loading.
(Vehicle) Live Loads (LL)
l Design truck
(Vehicle) Live Loads (LL)
l Design tandem
(Vehicle) Live Loads (LL)
l AASHTO Standard vs LRFD
(Vehicle) Live Loads (LL)
l Reduction for multiple lane presence
l A reduction in the live load intensity is permitted for bridges with two or more lanes that have maximum stress caused by fully loading each lane.
l AASHTO Standard Spec.
l 10% reduction for three lane loading
l 25% reduction for four or more lane loading
l AASHTO LRFD
l Multiple presence factor
l 20% increase for a single lane loading
l 15% reduction for three lane loading
l 35% reduction for four or more lane loading
Impact
(Dynamic Load Allowance)
l In order to account for the dynamic effect of a vehicle riding over a structure, an impact factor is used as a multiplier for certain structural elements.
l AASHTO Standard Spec.
l I=50/(L+125)≤0.3, L=length of span loaded to create maximum stress (ft)
l L for truck load moment = Design span length
l L for truck load shear = Length loaded portion of span from point of analysis to farthest reaction
l L for transverse members = Span length of member from center to center of supports (e.g., a floor beam)
l L for continuous span = Length of span being analyzed for positive moment and the average of two adjacent spans loaded for negative moment
l AASHTO LRFD
l 15% for fatigue and fracture limit states
l 33% for all other limit states
l 75% for deck joints for all limit states
l Only apply to the truck or tandem portion of live load, not to the lane load
Field Measurement of
Dynamic Effects
Dynamic Amplification Factors (DAF)
max( )
Um mm usmax (mm)
10 15 20 25 30 35 40 45
-3 -2 -1 0 1
Time (sec)
Displacement (mm)
Measured displacement (vision) Pseudo-static displacement (vision)
325 330 335 340 345 350 355 360 365
-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1
Time (sec)
Displacement (mm)
Measured displacement (vision) Pseudo-static displacement (vision)
636 638 640 642 644 646 648
-1 -0.5 0 0.5 1
Time (sec)
Displacement (mm)
Measured displacement (vision) Pseudo-static displacement (vision)
Case00 Case01 Case02
max max m s
DAF u
= u
Does each load act individually
or in various combination?
Group Loadings
(AASHTO Standard Spec.)
( )
( )
D L C E B
N S W WL L
R EQ ICE
D L I CF E B
Group SF W WL LF
R S T EQ ICE
b b b b b
g b b b b
b b b
é × + × + + × + × + × ù
ê ú
= ×ê+ × + × + × + × ú
ê+ × + + + × + × ú
ë û
where group number load factor coefficient
N g b
=
=
=
Group Loadings
(AASHTO Standard Spec.)
Table 3.2 p.114
기본적으로
fa=0.6fy, 1.5*fa=1.5*0.6fy=0.9fy, 1.7*fa=1.7*0.6fy=1.0fy
따라서1.25*fa=1.25*0.6fy=0.75fy, 1.33*fa=1.33*0.6fy=0.8fy, 1.40*fa=1.40*0.6fy=0.85fy
풍하중에 대한 활하중재하 풍하 중 조합p=0.5rho*B*Cd*U^2*L,
25m/s^2=625, 45m/s^2=2025, 625/2025=0.31
활하중계수 1.3*1.67=2.15 (구 도 로교설계기준)
IA 조합은 부반력 조합, 활하중을 2배로 재하하되 허용응력
150%(0.9fy)로 할증 검토
Earth Pressure and Dead Load Coefficients for LFD
Table 3.3 p.115
LRFD Design Philosophy
in which:
For loads for which a maximum value of is appropriate:
0.95
For loads for which a minimum value of is appropriate:
1 1.0
i i i n r
i
i D R I
i
i
D R I
Q R R h g f
g h h h h
g
h h h h
£ =
= ³
= £
å
Load Combinations for Limit States (AASHTO LRFD)
Table 3.4 p.117
Limit States in AASHTO LRFD
l
Strength limit state
l I : Basic load combination relating to the normal vehicular use
l II : Owner-specified special design vehicles or permit vehicles
l III : Being exposed to maximum wind velocity, no live load is assumed to be present on the bridge
l IV : For structures with very high dead to live load force effect ratios
l V : Normal vehicular use of the bridge with wind velocity of 25m/s(90km/h)
l
Extreme event limit state
l I : Related to earthquake
l II : Extreme events such as ice load, collision by vessels and vehicles
l
Service limit state
l I : Normal operational use of the bridge with a 25m/s(90km/h) wind
l II : Preventing yielding of steel structures due to vehicular live load
l III : Relates only to tension in prestressed concrete superstructure
l IV : Relates only to tension in prestressed concrete substructure to control cracks
l
Fatigue limit state
l Relates to repetitive gravitational vehicular live load and dynamic responses
l The live load factor 0.75 reflects a load level that represents the majority of truck population.
l Only a single truck with a constant spacing of 9.1m between 142KN axles
Load Factors for Permanent Loads (AASHTO LRFD)
Table 3.5 p.117
Also in the Korean design code
…
Dead Loads
Live Loads
Impact Factor
l 정적 하중에 적용시켜야 할 충격하중계수는 다음과 같다 : (1+IM/100)
참고: KBDC(2012) pp3-17
Application of Live Loads
Application of Live Loads
Application of Live Loads
Application of Live Loads
Application of Live Loads
Application of Live Loads
Application of Live Loads
Application of Live Loads
한계상태설계법
교량의 부재들과 연결부들은 다음의 각 한계상태에서 규정된 극한하중효과의 조합들에 대하여 식(3.4.1)에 의해 검토하여야 한다.
l 극한한계상태 하중조합 I : 일반적인 차량통행을 고려한 기본하중조합. 이때 풍하중 은 고려하지 않는다.
l 극한한계상태 하중조합 II : 발주자가 규정하는 특수차량이나 통행허가차량을 고려한 하중조합. 풍하중은 고려하지 않는다.
l 극한한계상태 하중조합 III : 풍속 90km/hr(25m/sec)를 초과하는 풍하중을 고려하는 하중조합.
l 극한한계상태 하중조합 IV : 활하중에 비하여 고정하중이 매우 큰 경우에 적용하는 하중조합.
l 극한한계상태 하중조합 V : 90km/hr의 풍속과 일상적인 차량통행에 의한 하중효과를 고려한 하중 조합.
l 극단상황한계상태 하중조합 I : 지진하중을 고려하는 하중조합.
l 극단상황한계상태 하중조합 II : 빙하중, 선박 또는 차량의 충돌하중 및 감소된 활하 중을 포함한 수리학적 사건에 관계된 하중조합. 이때 차량충돌하중 CT의 일부분인 활 하중은 제외된다.
한계상태설계법
l 사용한계상태 하중조합 I : 교량의 정상 운용 상태에서 발생 가능한 모든 하중의 표준 값과 25m/s의 풍하중을 조합한 하중상태이며, 교량의 설계 수명 동안 발생 확률이 매 우 적은 하중조합이다. 이 하중조합은 철근콘크리트의 사용성 검증에 사용할 수 있다.
또한 옹벽과 사면의 안정성 검증, 매설된 금속구조물, 터널라이닝판과 열가소성 파이 프에서의 변형제어에도 적용한다.
l 사용한계상태 하중조합 II : 차량하중에 의한 강구조물의 항복과 마찰이음부의 미끄러 짐에 대한 하중조합.
l 사용한계상태 하중조합 III : 교량의 정상 운용 상태에서 설계 수명 동안 종종 발생 가 능한 하중조합이다. 이 조합은 부착된 프리스트레스 강재가 배치된 상부구조의 균열폭 과 인장응력 크기를 검증하는데 사용한다.
l 사용한계상태 하중조합 IV : 설계수명 동안 종종 발생 가능한 하중조합으로 교량 특성 상 하부구조는 연직하중보다 수평하중에 노출될 때 더 위험하기 때문에 연직 활하중 대신에 수평 풍하중을 고려한 하중조합이다. 따라서 이 조합은 부착된 프리스트레스 강재가 배치된 하부구조의 사용성 검증에 사용해야 한다. 물론 하부구조는 사용하중조 합 III에서의 사용성 요구 조건도 동시에 만족하도록 설계하여야 한다.
l 피로한계상태 하중조합 : 3.6.2항에 규정되어 있는 피로설계트럭하중을 이용하여 반복 적인 차량하중과 동적응답에 의한 피로파괴를 검토하기 위한 하중조합.
